The assassin bug: aggressive mimicry of prey

I’m shamelessly stealing this story from Alex Wild’s great Scientific American website, Compound Eye. His latest post describes a paper from the Proceedings of the Royal Society B (link below) by Wignali and Taylor, who show that assassin bugs from Australia (Stenolemus bituberus; these are true bugs in the order Hemiptera) kill spiders by entering their webs and producing vibrations that lure the spider by mimicking either the vibrations made by normal prey trapped in the web. This shows that the evolution of mimicry need not involve any change in appearance but simply a change in behavior: in this case natural selection has favored those assassin bugs who are able to vibrate spider webs with the proper frequency.

Assassin bugs are usually called “thread-legged bugs” for obvious reasons:

Reduviidae: Emesinae (Belize); not the species used in the study but a related one. Photo used with permission.

They’re cryptic, too; as Alex notes: “In the field the insect looked like so little I thought it merely debris in a disorganized spider’s web. I didn’t see the faint outline of a young assassin bug until the debris shuddered, ever so slightly.” (Remember that some spiders, and of course bug-hunting birds, have keen vision.)

I hope my readers are now biology-savvy enough to understand the paper’s abstract:

Assassin bugs (Stenolemus bituberus) hunt web-building spiders by invading the web and plucking the silk to generate vibrations that lure the resident spider into striking range. To test whether vibrations generated by bugs aggressively mimic the vibrations generated by insect prey, we compared the responses of spiders to bugs with how they responded to prey, courting male spiders and leaves falling into the web. We also analysed the associated vibrations. Similar spider orientation and approach behaviours were observed in response to vibrations from bugs and prey, whereas different behaviours were observed in response to vibrations from male spiders and leaves. Peak frequency and duration of vibrations generated by bugs were similar to those generated by prey and courting males. Further, vibrations from bugs had a temporal structure and amplitude that were similar to vibrations generated by leg and body movements of prey and distinctly different to vibrations from courting males or leaves, or prey beating their wings. To be an effective predator, bugs do not need to mimic the full range of prey vibrations. Instead bugs are general mimics of a subset of prey vibrations that fall within the range of vibrations classified by spiders as ‘prey’.

Here’s another photo showing the bug entering a spider’s web for nefarious purposes:

Finally, a video (taken from the original paper via Alex) showing an assassin bug luring a spider to its death:

Some assassin bugs also kill spiders not by mimicking prey vibrations, but by sneaking up on them and stabbing them with their mouthparts (ergo their name). An earlier BBC report notes that, when using this latter tactic, assassin bugs are most likely to move toward their spider prey when the wind is blowing, masking any vibrations produced by their movement. They’re like ninja cats! This was demonstrated in clever experiments using fans to mimic the vibrations of spider webs produced by wind.

Point of terminology: assassin bugs are not called thread-legged bugs. Thread-legged bugs are one subfamily of assassin bugs, historically sometimes considered a separate family, and now apparently found to be polyphyletic.

I am not familiar with any taxonomy at all (although observing the many assassin bugs in my garden -Rhynocoris sp.- is a favourite pastime of mine), but I noticed that in the wikipedia entry for the Reduviidae family assassin bugs and thread-legged bugs (subfamily Emesinae) are mentioned as different subgroups, i.e. the latter according to that particular entry are not assassin bugs. Elsewhere all Reduviidae are called assassin bugs. Confusing.

While I recognise the ecological importance and fascinating behavioural range of spiders, I also still have my pre-adolescent visceral fear of them. So all I can say is “Go, Assassin Bugs, Go!”
I thought that the defining characteristic of “bugs” were the piercing mouth parts. But the order name “Hemiptera” suggests that the “half-wings” (split wing cases?) are the defining characteristic. Or is this just another case of terminology not actually keeping up with the taxonomic definitions?

Traditional hemipterans do have a sort of split wing: the forward/side half of the forewing is generally hard and armored, while the rear/center half is membranous, like a typical insect wing. But since the former order Homoptera has been folded into it, the name is no longer literally appropriate.

There must be a subject along the lines of “history of taxonomic mistakes”. The Geological Society has a “History of Geology” meetings group … so I suppose the people on the other side of the arch will have an equivalent too?

(“The Arch” : The Geological Society of London (former Secretary including one C.Darwin has it’s rooms on one side of the archway into Burlington House, Piccadilly, London ; the Linnean Society (where the joint Darwin-Wallace paper was read in 1858) forms the other side of the arch. Dad turns right on going through the arch ; I turn left.)

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